Chart filter for Charts not assigned to any Dashboard

[LyleScott]

To clean up Charts, we often want to find all Charts not associated with any Dashboard.

On the Charts List page, /chart/list/, a Dashboard filter is offered, but you can't select a 'not set' value. You also cannot sort by the Charts table by the Dashboards column as a work around to get all charts with no Dashboards assigned. Outside of writing a custom API script, you have to paginate and just look at the Dashboards column. We have 100+ pages of charts at times, so this doesn't scale well.

We are currently on Superset 4.1.1

[rusackas]

Hi there. This would be a helpful addition. I hate to leave the traditional open-source repo comment, but would you be open to contributing a PR? I agree with the idea, but bandwidth is limited, addressing other efforts.